Doornkop Gold Mine (Johannesburg)

 mine, gold mine

Doornkop Gold Mine is owned and operated by Harmony (one of the world’s 10 largest gold mining companies) and is situated 30 kilometres west of Johannesburg, comprises a single shaft system of intermediate depth (maximum depth of 1 973m) and is serviced by the Doornkop carbon-in-pulp (CIP) Gold Plant.

At Doornkop, both the Kimberley and South reefs are mined by means of narrow reef, conventional mining and mechanised bord and pillar mining. Production at the Doornkop South Reef Project, on which development began in FY03 and which enables access to the higher grade South Reef, began in FY08. The project is currently being ramped up, with full production scheduled for FY15.

In FY10, Doornkop employed 2 649 people – 1 645 employees and 1 004 contractors. At the end of June 2010, Doornkop reported mineral reserves of 0.437 million ounces and mineral resources of 21.603 million ounces.

Geology

The structure of the West Rand goldfield is dominated by the Witpoortjie and Panvlakte Horst blocks, which are superimposed over broad folding associated with the south-east plunging West Rand syncline. At the Doornkop mine, both the Kimberley Reef and the South Reef are exploited.

The Doornkop shaft lease area is bounded by and lies to the south-east of the major north-easterly striking Roodepoort Fault, which dips to the south and constitutes the southern edge of the Witpoortjie Horst Block or Gap. This Horst Block is comprised of the stratigraphically older sediments of the West Rand Group, the overlying Central Rand Group sediments having been removed by erosion. A number of other faults, forming part of and lying south-east of the Roodepoort Fault, including the Saxon Fault, also constitute conspicuous structural breaks. A second major fault, the Doornkop Fault, which trends in an east-west direction occurs towards the southern portion of the lease area. This fault dips to the south and has an up-throw to the north.

Nearly the entire upper Witwatersrand section is present in the Doornkop lease area and therefore all the major zones are present, though, due to the distance of the area from the fan head, the number of economic bands and their payability is limited. Eight of the well-known reefs are present in the area, but only the Kimberley Reef and South Reef are considered viable at this stage.

The resource is concentrated in the Kimberley and South Reefs. The Kimberley Reef is contained in the Vlakfontein member of the Westonaria Formation. This reef, also known as the K9 Reef horizon, rests on an unconformity and is a complex multi-pulse conglomerate, which can be separated into four facies or cycles. All four cycles consist on average of an upper conglomerate and a lower quartzite. The characteristics of every cycle are area-dependent and the grades are variable within each cycle.

The South Reef is approximately 900 metres below the current Kimberley Reef mining area, and between 7.5 and 60 metres above the Main Reef horizon. The hanging wall to the South Reef consists of siliceous quartzites with non-persistent bands of ’blueshot‘ grit and thin argillite partings. The footwall to the South Reef is a light coloured and fairly siliceous quartzite. Secondary conglomerate bands and stringers in the hanging wall and footwall of the South Reef may contain sporadic gold values.

The general strike of the reef is east-west, with a dip from 10 to 20 degrees. The orebody at Doornkop has a strike length of 4 kilometres and a width of 4 kilometres from west to east.

Operational review – FY10

Despite a 2% decline in volumes milled to 540 000 tonnes, an increase of 52% in grade contributed to gold produced being 49% up at 62 694 ounces year-on-year. The much improved grade was a result of the growth in high-grade volumes mined from the South Reef and under achievement on the low-grade Kimberley Reef as well as a high mine call factor for the shaft as a whole.

The greater volume of South Reef ore mined was in line with the build up of production at the South Reef project. Tonnes mined from South Reef mining areas accounted for 50% of total tonnes mined in FY10 – up from 30% the year before – while the contribution from the Kimberley Reef declined from 70% to 50%. The average grade of South Reef tonnes mined rose to 5.4g/t while that of the Kimberley Reef remained flat.

The low-grade Kimberley Reef operation is mechanised and production during the year was compromised by the lack of availability of trackless machinery. The new trackless fleet that has been purchased will improve production in the coming year.

There was a steady improvement in development metres achieved which will help to ensure that the build-up in the South Reef project is achieved and that targets are met. Problems were however encountered with the equipping of the shaft in terms of the South Reef project given the temporary shaft time constraints experienced. The winder compartments were subsequently equipped and the conveyor belt on 212 level (shaft bottom) was completed.

Much effort has been put into increasing our understanding of the geology of the South Reef orebody. This work has led to an increase in South Reef reserve ounces with the move in resources from inferred to indicated and measured categories, and has also increased confidence in the life-of-mine production plan. Further work to improve the geological model of the South reef continues.

There is currently a four-year plan in place for the mining of the Kimberley Reef in terms of which production areas on the Kimberley Reef will account for up to 75% of Doornkop’s total gold production annually. This proportion will decline as higher-grade volumes from the South Reef project build up. A new geological model has been developed of the Kimberley Reef to identify target areas for exploration, the results of which may extend the four-year plan currently in place.

The mine as a whole is building up to mine and process 1.6Mt annually.


Reference for information:
www.harmony.co.za/b/ops_sa_doornkop.asp